OSMOSIS AND DIFFUSION - Diffusion and osmosis are both passive transport processes that act to equalize the concentration of a solution. In diffusion, particles move from an area of higher concentration to one of lower concentration until equilibrium is reached. In osmosis, a semipermeable membrane is present, so only the solvent molecules are free to move to equalize concentration.

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Osmosis and Diffusion

What Is the Difference Between Osmosis and Diffusion?

by Anne Marie Helmenstine, Ph.D. 

Students are often asked to explain the similarities and differences between osmosis and diffusion or to compare and contrast the two forms of transport.

To answer the question, you need to know the definitions of osmosis and diffusion and really understand what they mean.

Osmosis and Diffusion Definitions

Osmosis: Osmosis is the movement of solvent particles across a semipermeable membrane from a dilute solution into a concentrated solution. The solvent moves to dilute the concentrated solution and equalize the concentration on both sides of the membrane.

Diffusion: Diffusion is the movement of particles from an area of higher concentration to lower concentration. The overall effect is to equalize concentration throughout the medium.

Osmosis and Diffusion Examples

Examples of Osmosis: Examples of osmosis include red blood cells swelling up when exposed to fresh water and plant root hairs taking up water. To see an easy demonstration of osmosis, soak gummy candies in water. The gel of the candies acts as a semipermeable membrane.

Examples of Diffusion: Examples of diffusion include perfume filling a whole room and the movement of small molecules across a cell membrane. One of the simplest demonstrations of diffusion is adding a drop of food coloring to water. Although other transport processes do occur, diffusion is the key player. 

More Examples of Diffusion

1.    Perfume is sprayed in one part of a room, yet soon it diffuses so that you can smell it everywhere.

2.    A drop of food coloring diffuses throughout the water in a glass so that, eventually, the entire glass will be colored.

3.    When steeping a cup of tea, molecules from the tea cross from the tea bag and diffuse throughout the cup of water.

4.    When shaking salt into water, the salt dissolves and the ions move until they are evenly distributed.

5.    After lighting a cigarette, the smoke spreads to all parts of a room.

6.    After placing a drop of food coloring onto a square of gelatin, the color will spread to a lighter color throughout the block.

7.    Carbon dioxide bubbles diffuse from an open soda, leaving it flat.

8.    If you place a wilted celery stick in water, water will diffuse into the plant, making it firm again.

9.    Water diffuses into cooking noodles, making them bigger and softer.

10.   A helium balloon deflates a little bit every day as helium diffuses through the balloon into the air.

11.   If you place a sugar cube in water, the sugar will dissolve and evenly sweeten the water without having to stir it.

Osmosis and Diffusion Similarities

Osmosis and diffusion are related processes that display similarities:

·       Both osmosis and diffusion equalize the concentration of two solutions.

·      Both diffusion and osmosis are passive transport processes, which means they do not require any input of extra energy to occur. In both diffusion and osmosis, particles move from an area of higher concentration to one of lower concentration.

Osmosis and Diffusion Differences

·      Diffusion can occur in any mixture, including one that includes a semipermeable membrane, while osmosis always occurs across a semipermeable membrane.

·      When people discuss osmosis in biology, it always refers to the movement of water. In chemistry, it's possible for other solvents to be involved. In biology, this is a difference between the two processes.

·      One big difference between osmosis and diffusion is that both solvent and solute particles are free to move in diffusion, but when we talk about osmosis, only the solvent molecules (water molecules) cross the membrane.

This can be confusing to understand because while the solvent particles are moving from higher to lower solvent concentration across the membrane, they are moving from lower to higher solute concentration (from a more dilute solution to a region of more concentrated solution). 

This occurs naturally because the system seeks balance or equilibrium. If the solute particles can't cross a barrier, the only way to equalize concentration on both sides of the membrane is for the solvent particles to move in. 

You can consider osmosis to be a special case of diffusion in which diffusion occurs across a semipermeable membrane and only the water or other solvent moves.

Table Comparing Diffusion Versus Osmosis

Diffusion	Osmosis
Any type of substance moves from area of highest energy or concentration to region of lowest energy or concentration.	Only water or another solvent moves from a region of high energy or concentration to a region of lower energy or concentration.
Diffusion can occur in any medium, whether it is liquid, solid, or gas.	Osmosis occurs only in a liquid medium.
Diffusion does not require a semipermeable membrane.	Osmosis requires a semipermeable membrane.
The concentration of the diffusion substance equalizes to fill the available space.	The concentration of the solvent does not become equal on both sides of the membrane.
Hydrostatic pressure and turgor pressure do not normally apply to diffusion.	Hydrostatic pressure and turgor pressure oppose osmosis.
Diffusion does not depend on solute potential, pressure potential, or water potential.	Osmosis depends on solute potential.
Diffusion mainly depends on the presence of other particles.	Osmosis mainly depends on the number of solute particles dissolved in the solvent.
Diffusion is a passive process.	Osmosis is a passive process.
The movement in diffusion is to equalize concentration (energy) throughout the system.	The movement in osmosis seeks to equalize solvent concentration (although it does not achieve this).

Key Points

·      Diffusion and osmosis are both passive transport processes that act to equalize the concentration of a solution.

·      In diffusion, particles move from an area of higher concentration to one of lower concentration until equilibrium is reached. In osmosis, a semipermeable membrane is present, so only the solvent molecules are free to move to equalize concentration.

Anne Marie Helmenstine, Ph.D.

Ph.D. in biomedical sciences from the University of Tennessee at Knoxville - Oak Ridge National Laboratory.

Science educator with experience teaching chemistry, biology, astronomy, and physics at the high school, college, and graduate levels.

ThoughtCo and About Education chemistry expert since 2001.

Widely-published graphic artist, responsible for printable periodic tables and other illustrations used in science.

Experience

Anne Helmenstine, Ph.D. has covered chemistry for ThoughtCo and About Education since 2001, and other sciences since 2013. She taught chemistry, biology, astronomy, and physics at the high school, college, and graduate levels. She has worked as a research scientist and also abstracting and indexing diverse scientific literature for the Department of Energy.

In addition to her work as a science writer, Dr. Helmenstine currently serves as a scientific consultant, specializing in problems requiring an interdisciplinary approach. Previously, she worked as a research scientist and college professor. 

Education

Dr. Helmenstine holds a Ph.D. in biomedical sciences from the University of Tennessee at Knoxville and a B.A. in physics and mathematics with a minor in chemistry from Hastings College. In her doctoral work, Dr. Helmenstine developed ultra-sensitive chemical detection and medical diagnostic tests.

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